Measurement of the energy expenditure of humans is important for a number of reasons, including the determination of the proper caloric content for feedings of hospitalized patients whose metabolisms may deviate from normal values, the monitoring of progress of weight loss diets to allow the adjustment of caloric inputs to achieve a target loss and the determination of energy expenditure during exercise.
A variety of indirect calorimeters for measuring oxygen consumption during respiration have been devised. One form of respiratory calorimeter, disclosed in my U.S. Pat. Nos. 4,917,108; 5,038,792; 5,179,985 and 5,178,155, measures the volume of a subject's inhalations over a period of time, and the volume of the subject's exhalations after carbon dioxide in the exhalations has been removed by an absorbent scrubber. These measurements are integrated over the time of measurement and the difference between the two summed volumes is a measure of the subject's oxygen consumption. This follows from the fact that inhaled oxygen is either absorbed into the blood in the subject's lungs or expelled during exhalation. Some portion of the blood absorbed oxygen is replaced with CO.sub.2. When the CO.sub.2 is removed from the exhaled volume, the summed difference between inhalation and exhalation volume over a period of time is equal to the absorbed oxygen.
In some versions of these prior calorimeters a capnometer was also used to measure the instantaneous value of the exhaled CO.sub.2 in a breath allowing the calculation of CO.sub.2 production, Resting Energy Expenditure (REE) and Respiratory Quotient (RQ).
The absorbent scrubber used with these previous systems, such as sodium hydroxide or calcium hydroxide, which reacts with the CO.sub.2 to form water plus a salt, has a limited ability to absorb CO.sub.2 and must be replenished after a period of use. The scrubber is also large and heavy relative to the other components of the calorimeter.